Standard ammunition cartridges for firearms are typically unitary in construction with the structural components of the cartridge being made from metal. In general, the cartridge includes a case of a generally cylindrical shape and terminated at a rearward end by a base having a rim. A propellant is contained in the case, and a primer is located in the base. A bullet or projectile is frictionally held in a forward end of the case. The case is sized to a particular caliber, which closely approximates the diameter of the projectile and is less than the diameter of the bore defined by the barrel of the firearm through which the projectile moves. When the cartridge is in battery located at a rearward end of the bore, operating the firearm causes the primer to be ignited (e.g., via a firing pin), which in turn ignites the propellant (usually gunpowder). Gases resulting from the ignition of the gunpowder result in an increase in pressure within the case, thereby causing the case to expand. Upon continued expansion of the case, the outer surface of the case seals against the wall of the firing chamber. Because the case cannot expand any further, there is a buildup of pressure in the case that causes ejecta to leave the case at its determined pressure so the projectile can achieve the correct velocity. The spent case is either removed manually or by the weapons operating system.
In commercial practice most ammunition is manufactured with drawn brass cases that are generally cylindrical and define a smooth outer circumferential surface that approximates the shape of the walls of the firing chamber. During firing of the cartridge, peak pressure is imparted to the case. The elasticity of the brass allows the case to expand diametrically under pressure and to contact the walls of the firing chamber forming a suitable seal in the firing chamber. In doing so, the engineered hoop strength of the material will not yield but will retain its original geometry through material memory. Once the pressure is relieved, the case returns to its original (or near original) condition. This quality, which is known as the “springback” of the case, facilitates the extraction of the case from the firing chamber. Without the case material exhibiting sufficient springback, the case would not return to its engineered taper, thereby resulting in increased friction at extraction and possibly malfunction.